When an engine works, lubricating oil is used to lubricate relevant parts of the engine. Therefore, a four-stroke engine is usually equipped with a lubricating oil tank to store the lubricating oil. The lubricating oil in the lubricating oil tank is supplied to the engine through an oil supply system. An oil suction device is usually disposed in the lubricating oil tank. The lubricating oil is outputted through the oil suction device. A small-sized four-stroke engine is usually used in existing gardening tools. It is also equipped with a lubricating oil tank and it also needs to add proper lubricating oil in the lubricating oil tank. Due to the special requirements of the gardening tools, the small-size four-stroke engine needs to work normally when it is used in a leant and even inverted manner. Therefore, the engine needs to work normally at any position; meanwhile, its lubricating system needs to work normally as well.
There are multiple lubricating systems on the existing small-size four-stroke engine provided, as well as multiple lubricating oil tank structures thereof. The typical one is a four-stroke engine disclosed in U.S. Pat. No. 6,213,079. As shown in FIG. 1, an oil-taking portion 40 is disposed in a lubricating oil tank 18; the oil-taking portion 40 includes a hose tube 42 and a back balance 43; an oil suction opening is disposed on the back balance 43; when the engine is leant or inverted, the back balance 43 will roll towards the leant or inverted direction; therefore, the back balance 43 will roll towards the accumulating direction of the lubricating oil, so as to keep the oil suction opening below the lubricating oil level. However, the structure has the defects that if there are other obstacles in the lubricating oil tank, for example, such structures as bulging portion, reinforcing rib and the like, the back balance 43 may be possibly obstructed by the obstacles, and cannot be immersed below the lubricating oil level, so that the lubricating oil cannot be sucked, which may cause damage to the engine.